Polygonum L. (knotgrasses) is a genus of annual and perennial herbs — occasionally subshrubs — in the family Polygonaceae, order Caryophyllales. Plants are characterised by much-branched stems, alternate leaves typically narrower than 2 cm, and distinctive membranous ochreae (tubular sheaths) at the nodes — a hallmark of the buckwheat family. Flowers are small and bisexual, with five tepals (rarely four), four to eight stamens, and two or three styles; fruits are three-sided achenes with a curved embryo.
The genus was established by Carl Linnaeus in his Species Plantarum of 1753 and originally encompassed a much broader assemblage that included what are now recognised as Persicaria, Fallopia, Reynoutria, and Fagopyrum. Molecular phylogenetic work in the late twentieth and early twenty-first centuries progressively segregated these into separate genera; the circumscription of Polygonum in its strict sense remains the subject of ongoing revision. In this restricted concept the genus contains roughly 130 accepted species (NCSU), though GBIF records approximately 455 child taxa across the full aggregate; the genus's closest relatives are Duma and Atraphaxis in tribe Polygoneae.
Polygonum in the broad sense is cosmopolitan, centred on northern temperate regions. Species occupy a wide range of habitats — disturbed ground, roadsides, and arable fields (especially P. aviculare), shallow water and wetlands (e.g. P. amphibium, P. hydropiper), and alpine zones (P. viviparum). Most species bloom from early summer into autumn, producing flowers in axillary clusters or terminal spike-like racemes. Several members are considered agricultural weeds in Europe and North America, while larvae of various Lepidoptera use the plants as hosts. Polygonum cognatum is harvested as a pot herb in Turkey, and multiple species have been exploited for food during famines.
Etymology
The genus name Polygonum derives from the Greek poly- (many) and gonu (knee or joint), an allusion to the conspicuously swollen, jointed stems that are characteristic of the plants. An alternative reading connects the second element to gonos (seed or offspring) combined with Latin foecundus (fecund), yielding a meaning of "many seeds" — an interpretation the Flora of North America treats as the grammatically more defensible construction. Linnaeus applied the name when he formally described the genus in Species Plantarum in 1753, and it has remained in use ever since despite repeated contraction of the genus's boundaries.
Taxonomy
Linnaeus described Polygonum in 1753 (Species Plantarum, Vol. 1, p. 359), originally circumscribing it broadly to include what are now placed in Fagopyrum (buckwheats), Fallopia (including climbing buckwheats), Persicaria (smartweeds), and Reynoutria (knotweeds such as Japanese knotweed). Through the twentieth century, many taxonomic treatments recognised several of these as distinct genera, and in SEINet's data the long-standing note that "most of the several sections have been taken by some authors as distinct genera" reflects this contested history.
Molecular phylogenetic studies have progressively confirmed that the broad Linnaean Polygonum is polyphyletic and that the segregates deserve generic recognition. In its current narrow circumscription Polygonum falls in tribe Polygoneae and is most closely related to Duma and Atraphaxis — the so-called "DAP clade". As of 2015 the circumscription was still described as ongoing. The genus is placed in Polygonaceae within Caryophyllales and contains around 130 accepted species in conservative treatments; GBIF records roughly 455 child taxa reflecting broader aggregate usage.
Distribution
Polygonum is a cosmopolitan genus centred on the northern temperate zone. Species are documented across Europe, Asia, North America, and beyond, with ecological niches ranging from coastal lowlands to alpine habitats. In Switzerland, around 20 species are recorded, spanning alpine conditions (P. alpinum, P. viviparum), aquatic and wetland settings (P. amphibium, P. hydropiper), and disturbed or ruderal ground (P. aviculare aggregate). In North America the genus is widespread in temperate regions, with species recorded across North Carolina's Coastal Plain, Piedmont, and Mountains, and throughout the Southwest from Arizona and New Mexico to the Great Plains and northeastern states. NCSU characterises the overall distribution as cosmopolitan.
Ecology
Most species of Polygonum are associated with disturbed, often moist environments — roadsides, cultivated ground, riverbanks, and shallow water. Native forms grow partially immersed in shallow waters of marshes, swamps, wet forests, and drainage ditches. Inflorescences are typically terminal or axillary spike-like racemes; flowering runs from early or midsummer into autumn. The plants provide cover and food for wildlife, and several Lepidoptera larvae use Polygonum species as hosts. The majority of species are treated as weeds in Europe and North America, particularly P. aviculare (common knotgrass), which colonises compacted soils along paths and in arable fields.
Cultivation
Polygonum species generally perform best in full sun (six or more hours of direct light daily) and prefer moist, well-drained soils, though many tolerate wet or even periodically inundated conditions. NCSU rates the genus as high-maintenance. Plants are hardy across a wide climatic range (USDA zones 4a–8a), making them suitable for temperate gardens where wet or riparian conditions exist. Because most species are considered weedy, cultivation is typically reserved for native and wildlife plantings rather than ornamental borders.
Propagation
Division is the recommended propagation method for perennial Polygonum species. Annual species are readily grown from seed.
Cultural uses
Several Polygonum species are edible and have been used as food, particularly in times of scarcity. Polygonum cognatum (known in Turkey as "madimak") is regularly eaten as a cooked green in central Anatolia and remains part of local culinary tradition. The genus appears in European literary culture in connection with knotgrass: Shakespeare's A Midsummer Night's Dream and Victor Hugo's The Man Who Laughs both reference the plant's purported ability to stunt growth when ingested in childhood.
Conservation
In Switzerland, Polygonum species have been assessed for the National Red List (2016) and Regional Red List (2019), indicating that some taxa face conservation concern at the national level. No genus-wide global conservation status is assigned; the majority of species are common and often weedy. The IUCN Global Invasive Species Database does not currently list Polygonum as an invasive taxon at the genus level.
